This invention relates to a process for preparing colored aluminum powders, and more particularly to a process for preparing a colored aluminum powder to be added to a coating composition as a pigment to give the composition a metallic color.
The term "aluminum" as used herein and in the claims includes pure aluminum, commercial aluminum containing small amounts of impurities and aluminum alloys in which aluminum predominates.
It is known to add finely divided metal to a coating composition as a pigment to prepare a coating composition having a metallic color. As such metal powder pigment, aluminum powder is used to obtain a silver color, or brass powder is used to give a gold color. Use of brass powder involves problems in that it is expensive, unusable for articles related to beverages and foods because it is harmful, prone to discoloration and corrosion depending on the environment, liable to delustering and subject to color change to gray at a temperature of 300.degree. to 500.degree. C.
Attempts have also been made to add colored aluminum powders to coating compositions to prepare compositions having varying metallic colors. For this purpose, various studies have been made on methods for coloring aluminum particles which mainly include two methods: one in which an oxide film formed on the surface of aluminum particles is colored with an organic dye, and the other in which a colored synthetic resin film is coated with aluminum by vacuum evaporation and the coated film is then comminuted. However, these methods are infeasible because the aluminum particles colored by the former method have not ful resistance to weather and corrosion nor sufficient brilliance, while the latter method requires a very expensive apparatus for vacuum evaporation coating and further produces colored aluminum particles of poor resistance to heat and weather.